680 



Fishery Bulletin 



1990 



absorption efficiences obtained using "natural" POM. 

 Clearance and ingestion rates of particulate material 

 were assumed to be independent of water temperature, 

 but dependent on mussel size and particle concentra- 

 tion (Foster-Smith 1975, Bayne et al. 1976b, Widdows 

 1978, Widdows et al. 1979). The fraction of filtered 

 seston rejected as pseudofeces (% pseudofeces. Table 

 1) was determined from ingestion rate and the critical 

 POM concentration (3.0 mg/L for a 20-mm mussel; 

 Widdows et al. 1979). The percent pseudofeces repre- 

 sents the fraction of filtered seston rejected as pseudo- 

 feces. Absorption efficiency was estimated using the 

 model of Bayne et al. (1979; Table 1). The caloric con- 

 tent of the absorbed food was assumed to be 23.5 

 Joules/mg dry POM (Widdows 1985b). 



Metabolic expenditures were estimated given mussel 

 size, water temperature, and published data on oxy- 

 gen consumption (Thompson 1984, Widdows et al. 

 1984; Table 1). Published oxygen consumption mea- 

 surements were converted to an energetic ecjuivalent 

 of 20.3 Joules/mL O2 (Crisp 1971). Energy losses due 

 to excretion are generally minor (as a percent of ab- 

 sorbed ration: 4.3-5.9%, Bayne et al. 1979; 1.7-4.3%, 

 Widdows et al. 1980; 0.4-2.0%, Thompson 1984; 

 0.5-2.9%, Widdows and Shick 1985) and were ignored 

 here. SFG (Joules/hour) was estimated as (absorbed 

 POM, mg/hour x 23.5 J/mg) - (mL O./hour x 20.3 

 J/mL 0.>). 



Figure 2 



(a) Water temperatures and (b) chlorophyll ti concen- 

 trations at Avila Beach (•) and Platform Holly (O). 



Results 



Physical and biological parameters 



Surface water temperature was significantly lower at 

 Avila than at Holly (/ = 2.46, df = 49, P<0.01, Student's 

 /-test; Fig. 2a). Water temperatures at Avila were up 

 to 3°C cooler than at Holly from October through 

 December 1986. 



Surface chlorophyll (/ concentration was significant- 

 ly higher at Avila "than at Holly (( = 3.53, df=49, P 

 < 0.001, Student's Mest; Fig. 2b). Chlorophyll a con- 

 centrations were as much as 20 times higher at Avila 

 than at Holly from October through November 1986. 



Seston concentration was higher at Avila than at 

 Holly (/ = 3.88, df = 46, P<0.001; Fig. 3a). The highest 

 values at Avila (10-30 mg/L) January through Febru- 

 ary coincided with seasonal storms. Seston levels re- 

 mained low at Holly, fluctuating between 1 and 5 mg/L 

 during most of the year. 



Particulate organic matter concentration was higher 

 at Avila than at Holly (/ = 2.04, df = 40, P<0.05; Fig. 

 3b). Particulate organic matter concentrations at Avila 

 generally varied between 1 and 5 mg/L, but reached 

 10 mg/L in January 1987. Particulate organic matter 



concentrations at Holly ranged from <0.5 mg/L to a 

 high value of 4 mg/L during a phytoplankton bloom in 

 March 1987. Differences in percent POM between the 

 two locations were most evident in January and Feb- 

 ruary 1987, when percent POM was 10-35%) at Avila 

 and 40-70% at Holly (Fig. 3c). 



Inarticulate organic carbon concentration was sig- 

 nificantly higher at Avila than at Holly (/ = 4.88, df 

 = 38, P< 0.001; Fig. 4). Values at Avila ranged from 

 ~400 ^g C/L to ~1400 ^Jg C/L, with values exceeding 

 1000 /.(g C/L in late November 1986, and January, 

 April, and June 1987. In contrast, POC concentrations 

 at Holly remained below 500 f^g C/L, except during 

 March- April 1987 when values of 650-900 /.(g C/L were 

 recorded. The peaks in POC concentration at Avila 

 coincided with elevated chlorophyll a concentrations in 

 November, April, and June, but not in January. The 

 peaks in POC concentration at Holly coincided with the 

 elevated chlorophyll n concentrations in March-April. 



The slopes of linear least-square regression lines of 

 POC concentration versus chlorophyll a concentration 

 at Avila and Holly were significantly different from 

 (P<0.05, ANOVA). There was no significant difference 



